There are broadly two types of image reading apparatuses, used in copiers, scanners, facsimiles, and the like, for reading the entire image by scanning an image at a reading position using a one-dimensional image pickup device. It should be noted that, in general, a direction in which one-dimensional image pickup devices are arranged is referred to as a main scanning direction, and a direction in which scanning is performed is referred to as sub-scanning direction.
In one of the two types, a single-eye lens is used to reduce and transfer the entire image in the main scanning direction onto image pickup devices. This system is mainly used by a copier to read the front side. In this system, the image pickup devices and the lens located at the side of a document are usually fixed, and only a mirror moves in the sub-scanning direction so as to scan the entire document. In this system, the depth of focus (or the depth of field) at the side of the document is rather large in the order of several millimeters, for example, 6 mm. Therefore, this system has an advantage in that the document can be read even when the document is not in close contact with the reading surface for the document of the copier. For example, even when a gutter of a bound book cannot come into contact with the reading surface for the document, this system has an advantage in that the document can be read without any defocus. Therefore, this system has been mainly used by copiers to read the front side. There are various patent documents derived from this method. For example, Patent Document 1 is listed below (hereinafter referred to as conventional method 1).
In the other of the two types, an image in the main scanning direction is divided into a plurality of parts and read by a compound-eye lens. This is usually called as a contact image sensor. This system is used by a copier which reads the back side, a facsimile machine which reads a document, a banknote recognition sensor, a scanner for personal computer, and the like, and is characterized by its small size. For example, Patent Document 2 discloses a conventional technique which has now become a mainstream of an optics system for this contact image sensor. Patent Document 2 discloses an image reading apparatus for obtaining an erect same-magnification-rate image by using a compound-eye lens (“rod lens array” in Patent Document 2) in which a plurality of rod lenses each having a distribution of refractive index defined by a certain function in a radius direction are arranged in a state of array (hereinafter referred to as conventional method 2).
Another typical exemplary system of optics system for the contact image sensor is, for example, a system disclosed in Patent Document 3. In this system, lenses are arranged respectively for cells divided in the main scanning direction, and images in regions respectively corresponding to the cells are reduced and transferred by the lenses onto image pickup devices so as to form images thereon. The output signals provided by the image pickup devices respectively arranged for the cells are combined into an image so as to restore the image existing on the document surface (hereinafter referred to as conventional method 3).
Patent Document 4 is similar to the above conventional method 2 or the above conventional method 3, and discloses a method for obtaining an erect same-magnification-rate image by using a compound-eye mirror lens array (hereinafter referred to as conventional method 4).
Patent Document 5 discloses a method for obtaining an erect same-magnification-rate image on an imaging plane, wherein a reading region is divided into odd-number regions and even-number regions, and an optical path of an imaging optics system is changed according to whether a region is an odd-number region or an even-number region, and wherein the imaging optics system is telecentric (hereinafter referred to as conventional method 5).
Further the applicants of the present application have already disclosed regarding an image reading apparatus with a large depth of field.    Patent Document 1: Japanese Patent Laid-Open No. H10-308852    Patent Document 2: Japanese Patent Laid-Open No. H8-204899    Patent Document 3: Japanese Patent Laid-Open No. H5-14600    Patent Document 4: Japanese Patent Laid-Open No. H11-8742    Patent Document 5: Japanese Patent Laid-Open No. 2005-37448    Patent Document 6: Japanese Patent Laid-Open No. 2009-246623